[crypto] Simplify internal HMAC API

Simplify the internal HMAC API so that the key is provided only at the
point of calling hmac_init(), and the (potentially reduced) key is
stored as part of the context for later use by hmac_final().

This simplifies the calling code, and avoids the need for callers such
as TLS to allocate a potentially variable length block in order to
retain a copy of the unmodified key.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

16 files changed, 142 insertions, 163 deletions

diff --git a/src/crypto/hmac.c b/src/crypto/hmac.c
index f898619..7109bbf 100644
--- a/src/crypto/hmac.c
+++ b/src/crypto/hmac.c
@@ -47,93 +47,61 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <ipxe/hmac.h>
 
 /**
- * Reduce HMAC key length
- *
- * @v digest		Digest algorithm to use
- * @v digest_ctx	Digest context
- * @v key		Key
- * @v key_len		Length of key
- */
-static void hmac_reduce_key ( struct digest_algorithm *digest,
-			      void *key, size_t *key_len ) {
-	uint8_t digest_ctx[digest->ctxsize];
-
-	digest_init ( digest, digest_ctx );
-	digest_update ( digest, digest_ctx, key, *key_len );
-	digest_final ( digest, digest_ctx, key );
-	*key_len = digest->digestsize;
-}
-
-/**
  * Initialise HMAC
  *
  * @v digest		Digest algorithm to use
- * @v digest_ctx	Digest context
+ * @v ctx		HMAC context
  * @v key		Key
  * @v key_len		Length of key
- *
- * The length of the key should be less than the block size of the
- * digest algorithm being used.  (If the key length is greater, it
- * will be replaced with its own digest, and key_len will be updated
- * accordingly).
  */
-void hmac_init ( struct digest_algorithm *digest, void *digest_ctx,
-		 void *key, size_t *key_len ) {
-	unsigned char k_ipad[digest->blocksize];
+void hmac_init ( struct digest_algorithm *digest, void *ctx, const void *key,
+		 size_t key_len ) {
+	hmac_context_t ( digest ) *hctx = ctx;
 	unsigned int i;
 
-	/* Reduce key if necessary */
-	if ( *key_len > sizeof ( k_ipad ) )
-		hmac_reduce_key ( digest, key, key_len );
-
 	/* Construct input pad */
-	memset ( k_ipad, 0, sizeof ( k_ipad ) );
-	memcpy ( k_ipad, key, *key_len );
-	for ( i = 0 ; i < sizeof ( k_ipad ) ; i++ ) {
-		k_ipad[i] ^= 0x36;
+	memset ( hctx->pad, 0, sizeof ( hctx->pad ) );
+	if ( key_len <= sizeof ( hctx->pad ) ) {
+		memcpy ( hctx->pad, key, key_len );
+	} else {
+		digest_init ( digest, hctx->ctx );
+		digest_update ( digest, hctx->ctx, key, key_len );
+		digest_final ( digest, hctx->ctx, hctx->pad );
+	}
+	for ( i = 0 ; i < sizeof ( hctx->pad ) ; i++ ) {
+		hctx->pad[i] ^= 0x36;
 	}
-	
+
 	/* Start inner hash */
-	digest_init ( digest, digest_ctx );
-	digest_update ( digest, digest_ctx, k_ipad, sizeof ( k_ipad ) );
+	digest_init ( digest, hctx->ctx );
+	digest_update ( digest, hctx->ctx, hctx->pad, sizeof ( hctx->pad ) );
 }
 
 /**
  * Finalise HMAC
  *
  * @v digest		Digest algorithm to use
- * @v digest_ctx	Digest context
- * @v key		Key
- * @v key_len		Length of key
+ * @v ctx		HMAC context
  * @v hmac		HMAC digest to fill in
- *
- * The length of the key should be less than the block size of the
- * digest algorithm being used.  (If the key length is greater, it
- * will be replaced with its own digest, and key_len will be updated
- * accordingly).
  */
-void hmac_final ( struct digest_algorithm *digest, void *digest_ctx,
-		  void *key, size_t *key_len, void *hmac ) {
-	unsigned char k_opad[digest->blocksize];
+void hmac_final ( struct digest_algorithm *digest, void *ctx, void *hmac ) {
+	hmac_context_t ( digest ) *hctx = ctx;
 	unsigned int i;
 
-	/* Reduce key if necessary */
-	if ( *key_len > sizeof ( k_opad ) )
-		hmac_reduce_key ( digest, key, key_len );
-
-	/* Construct output pad */
-	memset ( k_opad, 0, sizeof ( k_opad ) );
-	memcpy ( k_opad, key, *key_len );
-	for ( i = 0 ; i < sizeof ( k_opad ) ; i++ ) {
-		k_opad[i] ^= 0x5c;
+	/* Construct output pad from input pad */
+	for ( i = 0 ; i < sizeof ( hctx->pad ) ; i++ ) {
+		hctx->pad[i] ^= 0x6a;
 	}
-	
+
 	/* Finish inner hash */
-	digest_final ( digest, digest_ctx, hmac );
+	digest_final ( digest, hctx->ctx, hmac );
 
 	/* Perform outer hash */
-	digest_init ( digest, digest_ctx );
-	digest_update ( digest, digest_ctx, k_opad, sizeof ( k_opad ) );
-	digest_update ( digest, digest_ctx, hmac, digest->digestsize );
-	digest_final ( digest, digest_ctx, hmac );
+	digest_init ( digest, hctx->ctx );
+	digest_update ( digest, hctx->ctx, hctx->pad, sizeof ( hctx->pad ) );
+	digest_update ( digest, hctx->ctx, hmac, digest->digestsize );
+	digest_final ( digest, hctx->ctx, hmac );
+
+	/* Erase output pad (from which the key may be derivable) */
+	memset ( hctx->pad, 0, sizeof ( hctx->pad ) );
 }
diff --git a/src/crypto/hmac_drbg.c b/src/crypto/hmac_drbg.c
index 0982977..57bde4d 100644
--- a/src/crypto/hmac_drbg.c
+++ b/src/crypto/hmac_drbg.c
@@ -79,7 +79,7 @@ static void hmac_drbg_update_key ( struct digest_algorithm *hash,
 				   struct hmac_drbg_state *state,
 				   const void *data, size_t len,
 				   const uint8_t single ) {
-	uint8_t context[ hash->ctxsize ];
+	uint8_t context[ hmac_ctxsize ( hash ) ];
 	size_t out_len = hash->digestsize;
 
 	DBGC ( state, "HMAC_DRBG_%s %p provided data :\n", hash->name, state );
@@ -92,13 +92,11 @@ static void hmac_drbg_update_key ( struct digest_algorithm *hash,
 	assert ( ( single == 0x00 ) || ( single == 0x01 ) );
 
 	/* K = HMAC ( K, V || single || provided_data ) */
-	hmac_init ( hash, context, state->key, &out_len );
-	assert ( out_len == hash->digestsize );
+	hmac_init ( hash, context, state->key, out_len );
 	hmac_update ( hash, context, state->value, out_len );
 	hmac_update ( hash, context, &single, sizeof ( single ) );
 	hmac_update ( hash, context, data, len );
-	hmac_final ( hash, context, state->key, &out_len, state->key );
-	assert ( out_len == hash->digestsize );
+	hmac_final ( hash, context, state->key );
 
 	DBGC ( state, "HMAC_DRBG_%s %p K = HMAC ( K, V || %#02x || "
 	       "provided_data ) :\n", hash->name, state, single );
@@ -122,7 +120,7 @@ static void hmac_drbg_update_key ( struct digest_algorithm *hash,
  */
 static void hmac_drbg_update_value ( struct digest_algorithm *hash,
 				     struct hmac_drbg_state *state ) {
-	uint8_t context[ hash->ctxsize ];
+	uint8_t context[ hmac_ctxsize ( hash ) ];
 	size_t out_len = hash->digestsize;
 
 	/* Sanity checks */
@@ -130,11 +128,9 @@ static void hmac_drbg_update_value ( struct digest_algorithm *hash,
 	assert ( state != NULL );
 
 	/* V = HMAC ( K, V ) */
-	hmac_init ( hash, context, state->key, &out_len );
-	assert ( out_len == hash->digestsize );
+	hmac_init ( hash, context, state->key, out_len );
 	hmac_update ( hash, context, state->value, out_len );
-	hmac_final ( hash, context, state->key, &out_len, state->value );
-	assert ( out_len == hash->digestsize );
+	hmac_final ( hash, context, state->value );
 
 	DBGC ( state, "HMAC_DRBG_%s %p V = HMAC ( K, V ) :\n",
 	       hash->name, state );
diff --git a/src/crypto/ntlm.c b/src/crypto/ntlm.c
index 870af21..fb120f8 100644
--- a/src/crypto/ntlm.c
+++ b/src/crypto/ntlm.c
@@ -117,10 +117,9 @@ void ntlm_key ( const char *domain, const char *username,
 	struct digest_algorithm *md5 = &md5_algorithm;
 	union {
 		uint8_t md4[MD4_CTX_SIZE];
-		uint8_t md5[MD5_CTX_SIZE];
+		uint8_t md5[ MD5_CTX_SIZE + MD5_BLOCK_SIZE ];
 	} ctx;
 	uint8_t digest[MD4_DIGEST_SIZE];
-	size_t digest_len;
 	uint8_t c;
 	uint16_t wc;
 
@@ -141,8 +140,7 @@ void ntlm_key ( const char *domain, const char *username,
 	digest_final ( md4, ctx.md4, digest );
 
 	/* Construct HMAC-MD5 of (Unicode) upper-case username */
-	digest_len = sizeof ( digest );
-	hmac_init ( md5, ctx.md5, digest, &digest_len );
+	hmac_init ( md5, ctx.md5, digest, sizeof ( digest ) );
 	while ( ( c = *(username++) ) ) {
 		wc = cpu_to_le16 ( toupper ( c ) );
 		hmac_update ( md5, ctx.md5, &wc, sizeof ( wc ) );
@@ -151,7 +149,7 @@ void ntlm_key ( const char *domain, const char *username,
 		wc = cpu_to_le16 ( c );
 		hmac_update ( md5, ctx.md5, &wc, sizeof ( wc ) );
 	}
-	hmac_final ( md5, ctx.md5, digest, &digest_len, key->raw );
+	hmac_final ( md5, ctx.md5, key->raw );
 	DBGC ( key, "NTLM key:\n" );
 	DBGC_HDA ( key, 0, key, sizeof ( *key ) );
 }
@@ -170,8 +168,7 @@ void ntlm_response ( struct ntlm_challenge_info *info, struct ntlm_key *key,
 		     struct ntlm_nt_response *nt ) {
 	struct digest_algorithm *md5 = &md5_algorithm;
 	struct ntlm_nonce tmp_nonce;
-	uint8_t ctx[MD5_CTX_SIZE];
-	size_t key_len = sizeof ( *key );
+	uint8_t ctx[ MD5_CTX_SIZE + MD5_BLOCK_SIZE ];
 	unsigned int i;
 
 	/* Generate random nonce, if needed */
@@ -183,10 +180,10 @@ void ntlm_response ( struct ntlm_challenge_info *info, struct ntlm_key *key,
 
 	/* Construct LAN Manager response */
 	memcpy ( &lm->nonce, nonce, sizeof ( lm->nonce ) );
-	hmac_init ( md5, ctx, key->raw, &key_len );
+	hmac_init ( md5, ctx, key->raw, sizeof ( *key ) );
 	hmac_update ( md5, ctx, info->nonce, sizeof ( *info->nonce ) );
 	hmac_update ( md5, ctx, &lm->nonce, sizeof ( lm->nonce ) );
-	hmac_final ( md5, ctx, key->raw, &key_len, lm->digest );
+	hmac_final ( md5, ctx, lm->digest );
 	DBGC ( key, "NTLM LAN Manager response:\n" );
 	DBGC_HDA ( key, 0, lm, sizeof ( *lm ) );
 
@@ -195,14 +192,14 @@ void ntlm_response ( struct ntlm_challenge_info *info, struct ntlm_key *key,
 	nt->version = NTLM_VERSION_NTLMV2;
 	nt->high = NTLM_VERSION_NTLMV2;
 	memcpy ( &nt->nonce, nonce, sizeof ( nt->nonce ) );
-	hmac_init ( md5, ctx, key->raw, &key_len );
+	hmac_init ( md5, ctx, key->raw, sizeof ( *key ) );
 	hmac_update ( md5, ctx, info->nonce, sizeof ( *info->nonce ) );
 	hmac_update ( md5, ctx, &nt->version,
 		      ( sizeof ( *nt ) -
 			offsetof ( typeof ( *nt ), version ) ) );
 	hmac_update ( md5, ctx, info->target, info->len );
 	hmac_update ( md5, ctx, &nt->zero, sizeof ( nt->zero ) );
-	hmac_final ( md5, ctx, key->raw, &key_len, nt->digest );
+	hmac_final ( md5, ctx, nt->digest );
 	DBGC ( key, "NTLM NT response prefix:\n" );
 	DBGC_HDA ( key, 0, nt, sizeof ( *nt ) );
 }
diff --git a/src/crypto/sha1extra.c b/src/crypto/sha1extra.c
index cec0d35..9e296eb 100644
--- a/src/crypto/sha1extra.c
+++ b/src/crypto/sha1extra.c
@@ -49,7 +49,7 @@ void prf_sha1 ( const void *key, size_t key_len, const char *label,
 	u8 in[strlen ( label ) + 1 + data_len + 1]; /* message to HMAC */
 	u8 *in_blknr;		/* pointer to last byte of in, block number */
 	u8 out[SHA1_DIGEST_SIZE]; /* HMAC-SHA1 result */
-	u8 sha1_ctx[SHA1_CTX_SIZE]; /* SHA1 context */
+	u8 ctx[SHA1_CTX_SIZE + SHA1_BLOCK_SIZE]; /* HMAC-SHA1 context */
 	const size_t label_len = strlen ( label );
 
 	/* The HMAC-SHA-1 is calculated using the given key on the
@@ -65,9 +65,9 @@ void prf_sha1 ( const void *key, size_t key_len, const char *label,
 	for ( blk = 0 ;; blk++ ) {
 		*in_blknr = blk;
 
-		hmac_init ( &sha1_algorithm, sha1_ctx, keym, &key_len );
-		hmac_update ( &sha1_algorithm, sha1_ctx, in, sizeof ( in ) );
-		hmac_final ( &sha1_algorithm, sha1_ctx, keym, &key_len, out );
+		hmac_init ( &sha1_algorithm, ctx, keym, key_len );
+		hmac_update ( &sha1_algorithm, ctx, in, sizeof ( in ) );
+		hmac_final ( &sha1_algorithm, ctx, out );
 
 		if ( prf_len <= sizeof ( out ) ) {
 			memcpy ( prf, out, prf_len );
@@ -100,7 +100,7 @@ static void pbkdf2_sha1_f ( const void *passphrase, size_t pass_len,
 	u8 pass[pass_len];	/* modifiable passphrase */
 	u8 in[salt_len + 4];	/* input buffer to first round */
 	u8 last[SHA1_DIGEST_SIZE]; /* output of round N, input of N+1 */
-	u8 sha1_ctx[SHA1_CTX_SIZE];
+	u8 ctx[SHA1_CTX_SIZE + SHA1_BLOCK_SIZE];
 	u8 *next_in = in;	/* changed to `last' after first round */
 	int next_size = sizeof ( in );
 	int i;
@@ -114,9 +114,9 @@ static void pbkdf2_sha1_f ( const void *passphrase, size_t pass_len,
 	memset ( block, 0, sizeof ( last ) );
 
 	for ( i = 0; i < iterations; i++ ) {
-		hmac_init ( &sha1_algorithm, sha1_ctx, pass, &pass_len );
-		hmac_update ( &sha1_algorithm, sha1_ctx, next_in, next_size );
-		hmac_final ( &sha1_algorithm, sha1_ctx, pass, &pass_len, last );
+		hmac_init ( &sha1_algorithm, ctx, pass, pass_len );
+		hmac_update ( &sha1_algorithm, ctx, next_in, next_size );
+		hmac_final ( &sha1_algorithm, ctx, last );
 
 		for ( j = 0; j < sizeof ( last ); j++ ) {
 			block[j] ^= last[j];
diff --git a/src/include/ipxe/hmac.h b/src/include/ipxe/hmac.h
index 09d3e27..cf9d086 100644
--- a/src/include/ipxe/hmac.h
+++ b/src/include/ipxe/hmac.h
@@ -10,23 +10,45 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #include <ipxe/crypto.h>
 
+/** HMAC context type */
+#define hmac_context_t( digest ) struct {				\
+		/** Digest context */					\
+		uint8_t ctx[ digest->ctxsize ];				\
+		/** HMAC input/output padding */			\
+		uint8_t pad[ digest->blocksize ];			\
+	} __attribute__ (( packed ))
+
+/**
+ * Calculate HMAC context size
+ *
+ * @v digest		Digest algorithm to use
+ * @ret len		HMAC context size
+ */
+static inline __attribute__ (( always_inline )) size_t
+hmac_ctxsize ( struct digest_algorithm *digest ) {
+	hmac_context_t ( digest ) *hctx;
+
+	return sizeof ( *hctx );
+}
+
 /**
  * Update HMAC
  *
  * @v digest		Digest algorithm to use
- * @v digest_ctx	Digest context
+ * @v ctx		HMAC context
  * @v data		Data
  * @v len		Length of data
  */
-static inline void hmac_update ( struct digest_algorithm *digest,
-				 void *digest_ctx, const void *data,
-				 size_t len ) {
-	digest_update ( digest, digest_ctx, data, len );
+static inline void hmac_update ( struct digest_algorithm *digest, void *ctx,
+				 const void *data, size_t len ) {
+	hmac_context_t ( digest ) *hctx = ctx;
+
+	digest_update ( digest, hctx->ctx, data, len );
 }
 
-extern void hmac_init ( struct digest_algorithm *digest, void *digest_ctx,
-			void *key, size_t *key_len );
-extern void hmac_final ( struct digest_algorithm *digest, void *digest_ctx,
-			 void *key, size_t *key_len, void *hmac );
+extern void hmac_init ( struct digest_algorithm *digest, void *ctx,
+			const void *key, size_t key_len );
+extern void hmac_final ( struct digest_algorithm *digest, void *ctx,
+			 void *hmac );
 
 #endif /* _IPXE_HMAC_H */
diff --git a/src/include/ipxe/md4.h b/src/include/ipxe/md4.h
index 8f172e6..9f6cb8a 100644
--- a/src/include/ipxe/md4.h
+++ b/src/include/ipxe/md4.h
@@ -65,6 +65,9 @@ struct md4_context {
 /** MD4 context size */
 #define MD4_CTX_SIZE sizeof ( struct md4_context )
 
+/** MD4 block size */
+#define MD4_BLOCK_SIZE sizeof ( union md4_block )
+
 /** MD4 digest size */
 #define MD4_DIGEST_SIZE sizeof ( struct md4_digest )
 
diff --git a/src/include/ipxe/md5.h b/src/include/ipxe/md5.h
index 05c3974..527ad36 100644
--- a/src/include/ipxe/md5.h
+++ b/src/include/ipxe/md5.h
@@ -65,6 +65,9 @@ struct md5_context {
 /** MD5 context size */
 #define MD5_CTX_SIZE sizeof ( struct md5_context )
 
+/** MD5 block size */
+#define MD5_BLOCK_SIZE sizeof ( union md5_block )
+
 /** MD5 digest size */
 #define MD5_DIGEST_SIZE sizeof ( struct md5_digest )
 
diff --git a/src/include/ipxe/sha1.h b/src/include/ipxe/sha1.h
index a97035e..9cbbebd 100644
--- a/src/include/ipxe/sha1.h
+++ b/src/include/ipxe/sha1.h
@@ -65,6 +65,9 @@ struct sha1_context {
 /** SHA-1 context size */
 #define SHA1_CTX_SIZE sizeof ( struct sha1_context )
 
+/** SHA-1 block size */
+#define SHA1_BLOCK_SIZE sizeof ( union sha1_block )
+
 /** SHA-1 digest size */
 #define SHA1_DIGEST_SIZE sizeof ( struct sha1_digest )
 
diff --git a/src/include/ipxe/sha256.h b/src/include/ipxe/sha256.h
index e234cce..f226ad0 100644
--- a/src/include/ipxe/sha256.h
+++ b/src/include/ipxe/sha256.h
@@ -70,6 +70,9 @@ struct sha256_context {
 /** SHA-256 context size */
 #define SHA256_CTX_SIZE sizeof ( struct sha256_context )
 
+/** SHA-256 block size */
+#define SHA256_BLOCK_SIZE sizeof ( union sha256_block )
+
 /** SHA-256 digest size */
 #define SHA256_DIGEST_SIZE sizeof ( struct sha256_digest )
 
diff --git a/src/include/ipxe/sha512.h b/src/include/ipxe/sha512.h
index 8e22d83..82a9e4e 100644
--- a/src/include/ipxe/sha512.h
+++ b/src/include/ipxe/sha512.h
@@ -72,6 +72,9 @@ struct sha512_context {
 /** SHA-512 context size */
 #define SHA512_CTX_SIZE sizeof ( struct sha512_context )
 
+/** SHA-512 block size */
+#define SHA512_BLOCK_SIZE sizeof ( union sha512_block )
+
 /** SHA-512 digest size */
 #define SHA512_DIGEST_SIZE sizeof ( struct sha512_digest )
 
diff --git a/src/net/80211/wpa_ccmp.c b/src/net/80211/wpa_ccmp.c
index a073c6a..0abd217 100644
--- a/src/net/80211/wpa_ccmp.c
+++ b/src/net/80211/wpa_ccmp.c
@@ -478,16 +478,15 @@ struct net80211_crypto ccmp_crypto __net80211_crypto = {
 static void ccmp_kie_mic ( const void *kck, const void *msg, size_t len,
 			   void *mic )
 {
-	u8 sha1_ctx[SHA1_CTX_SIZE];
+	u8 ctx[SHA1_CTX_SIZE + SHA1_BLOCK_SIZE];
 	u8 kckb[16];
 	u8 hash[SHA1_DIGEST_SIZE];
-	size_t kck_len = 16;
 
-	memcpy ( kckb, kck, kck_len );
+	memcpy ( kckb, kck, sizeof ( kckb ) );
 
-	hmac_init ( &sha1_algorithm, sha1_ctx, kckb, &kck_len );
-	hmac_update ( &sha1_algorithm, sha1_ctx, msg, len );
-	hmac_final ( &sha1_algorithm, sha1_ctx, kckb, &kck_len, hash );
+	hmac_init ( &sha1_algorithm, ctx, kckb, sizeof ( kckb ) );
+	hmac_update ( &sha1_algorithm, ctx, msg, len );
+	hmac_final ( &sha1_algorithm, ctx, hash );
 
 	memcpy ( mic, hash, 16 );
 }
diff --git a/src/net/80211/wpa_tkip.c b/src/net/80211/wpa_tkip.c
index 3b1934b..3bd6515 100644
--- a/src/net/80211/wpa_tkip.c
+++ b/src/net/80211/wpa_tkip.c
@@ -545,15 +545,14 @@ struct net80211_crypto tkip_crypto __net80211_crypto = {
 static void tkip_kie_mic ( const void *kck, const void *msg, size_t len,
 			   void *mic )
 {
-	uint8_t ctx[MD5_CTX_SIZE];
+	uint8_t ctx[MD5_CTX_SIZE + MD5_BLOCK_SIZE];
 	u8 kckb[16];
-	size_t kck_len = 16;
 
-	memcpy ( kckb, kck, kck_len );
+	memcpy ( kckb, kck, sizeof ( kckb ) );
 
-	hmac_init ( &md5_algorithm, ctx, kckb, &kck_len );
+	hmac_init ( &md5_algorithm, ctx, kckb, sizeof ( kckb ) );
 	hmac_update ( &md5_algorithm, ctx, msg, len );
-	hmac_final ( &md5_algorithm, ctx, kckb, &kck_len, mic );
+	hmac_final ( &md5_algorithm, ctx, mic );
 }
 
 /**
diff --git a/src/net/pccrc.c b/src/net/pccrc.c
index 4cd82cd..a94bc0e 100644
--- a/src/net/pccrc.c
+++ b/src/net/pccrc.c
@@ -104,9 +104,8 @@ static void peerdist_info_segment_hash ( struct peerdist_info_segment *segment,
 					 const void *hash, const void *secret ){
 	const struct peerdist_info *info = segment->info;
 	struct digest_algorithm *digest = info->digest;
-	uint8_t ctx[digest->ctxsize];
+	uint8_t ctx[ hmac_ctxsize ( digest ) ];
 	size_t digestsize = info->digestsize;
-	size_t secretsize = digestsize;
 	static const uint16_t magic[] = PEERDIST_SEGMENT_ID_MAGIC;
 
 	/* Sanity check */
@@ -121,12 +120,10 @@ static void peerdist_info_segment_hash ( struct peerdist_info_segment *segment,
 	memcpy ( segment->secret, secret, digestsize );
 
 	/* Calculate segment identifier */
-	hmac_init ( digest, ctx, segment->secret, &secretsize );
-	assert ( secretsize == digestsize );
+	hmac_init ( digest, ctx, segment->secret, digestsize );
 	hmac_update ( digest, ctx, segment->hash, digestsize );
 	hmac_update ( digest, ctx, magic, sizeof ( magic ) );
-	hmac_final ( digest, ctx, segment->secret, &secretsize, segment->id );
-	assert ( secretsize == digestsize );
+	hmac_final ( digest, ctx, segment->id );
 }
 
 /******************************************************************************
diff --git a/src/net/tls.c b/src/net/tls.c
index 3c41444..21f7073 100644
--- a/src/net/tls.c
+++ b/src/net/tls.c
@@ -458,17 +458,17 @@ static int tls_generate_random ( struct tls_connection *tls,
  * Update HMAC with a list of ( data, len ) pairs
  *
  * @v digest		Hash function to use
- * @v digest_ctx	Digest context
+ * @v ctx		HMAC context
  * @v args		( data, len ) pairs of data, terminated by NULL
  */
 static void tls_hmac_update_va ( struct digest_algorithm *digest,
-				 void *digest_ctx, va_list args ) {
+				 void *ctx, va_list args ) {
 	void *data;
 	size_t len;
 
 	while ( ( data = va_arg ( args, void * ) ) ) {
 		len = va_arg ( args, size_t );
-		hmac_update ( digest, digest_ctx, data, len );
+		hmac_update ( digest, ctx, data, len );
 	}
 }
 
@@ -485,43 +485,38 @@ static void tls_hmac_update_va ( struct digest_algorithm *digest,
  */
 static void tls_p_hash_va ( struct tls_connection *tls,
 			    struct digest_algorithm *digest,
-			    void *secret, size_t secret_len,
+			    const void *secret, size_t secret_len,
 			    void *out, size_t out_len,
 			    va_list seeds ) {
-	uint8_t secret_copy[secret_len];
-	uint8_t digest_ctx[digest->ctxsize];
-	uint8_t digest_ctx_partial[digest->ctxsize];
+	uint8_t ctx[ hmac_ctxsize ( digest ) ];
+	uint8_t ctx_partial[ sizeof ( ctx ) ];
 	uint8_t a[digest->digestsize];
 	uint8_t out_tmp[digest->digestsize];
 	size_t frag_len = digest->digestsize;
 	va_list tmp;
 
-	/* Copy the secret, in case HMAC modifies it */
-	memcpy ( secret_copy, secret, secret_len );
-	secret = secret_copy;
 	DBGC2 ( tls, "TLS %p %s secret:\n", tls, digest->name );
 	DBGC2_HD ( tls, secret, secret_len );
 
 	/* Calculate A(1) */
-	hmac_init ( digest, digest_ctx, secret, &secret_len );
+	hmac_init ( digest, ctx, secret, secret_len );
 	va_copy ( tmp, seeds );
-	tls_hmac_update_va ( digest, digest_ctx, tmp );
+	tls_hmac_update_va ( digest, ctx, tmp );
 	va_end ( tmp );
-	hmac_final ( digest, digest_ctx, secret, &secret_len, a );
+	hmac_final ( digest, ctx, a );
 	DBGC2 ( tls, "TLS %p %s A(1):\n", tls, digest->name );
 	DBGC2_HD ( tls, &a, sizeof ( a ) );
 
 	/* Generate as much data as required */
 	while ( out_len ) {
 		/* Calculate output portion */
-		hmac_init ( digest, digest_ctx, secret, &secret_len );
-		hmac_update ( digest, digest_ctx, a, sizeof ( a ) );
-		memcpy ( digest_ctx_partial, digest_ctx, digest->ctxsize );
+		hmac_init ( digest, ctx, secret, secret_len );
+		hmac_update ( digest, ctx, a, sizeof ( a ) );
+		memcpy ( ctx_partial, ctx, sizeof ( ctx_partial ) );
 		va_copy ( tmp, seeds );
-		tls_hmac_update_va ( digest, digest_ctx, tmp );
+		tls_hmac_update_va ( digest, ctx, tmp );
 		va_end ( tmp );
-		hmac_final ( digest, digest_ctx,
-			     secret, &secret_len, out_tmp );
+		hmac_final ( digest, ctx, out_tmp );
 
 		/* Copy output */
 		if ( frag_len > out_len )
@@ -531,8 +526,7 @@ static void tls_p_hash_va ( struct tls_connection *tls,
 		DBGC2_HD ( tls, out, frag_len );
 
 		/* Calculate A(i) */
-		hmac_final ( digest, digest_ctx_partial,
-			     secret, &secret_len, a );
+		hmac_final ( digest, ctx_partial, a );
 		DBGC2 ( tls, "TLS %p %s A(n):\n", tls, digest->name );
 		DBGC2_HD ( tls, &a, sizeof ( a ) );
 
@@ -551,13 +545,13 @@ static void tls_p_hash_va ( struct tls_connection *tls,
  * @v out_len		Length of output buffer
  * @v ...		( data, len ) pairs of seed data, terminated by NULL
  */
-static void tls_prf ( struct tls_connection *tls, void *secret,
+static void tls_prf ( struct tls_connection *tls, const void *secret,
 		      size_t secret_len, void *out, size_t out_len, ... ) {
 	va_list seeds;
 	va_list tmp;
 	size_t subsecret_len;
-	void *md5_secret;
-	void *sha1_secret;
+	const void *md5_secret;
+	const void *sha1_secret;
 	uint8_t buf[out_len];
 	unsigned int i;
 
@@ -2213,7 +2207,7 @@ static void tls_hmac_init ( struct tls_cipherspec *cipherspec, void *ctx,
 			    uint64_t seq, struct tls_header *tlshdr ) {
 	struct digest_algorithm *digest = cipherspec->suite->digest;
 
-	hmac_init ( digest, ctx, cipherspec->mac_secret, &digest->digestsize );
+	hmac_init ( digest, ctx, cipherspec->mac_secret, digest->digestsize );
 	seq = cpu_to_be64 ( seq );
 	hmac_update ( digest, ctx, &seq, sizeof ( seq ) );
 	hmac_update ( digest, ctx, tlshdr, sizeof ( *tlshdr ) );
@@ -2245,8 +2239,7 @@ static void tls_hmac_final ( struct tls_cipherspec *cipherspec, void *ctx,
 			     void *hmac ) {
 	struct digest_algorithm *digest = cipherspec->suite->digest;
 
-	hmac_final ( digest, ctx, cipherspec->mac_secret,
-		     &digest->digestsize, hmac );
+	hmac_final ( digest, ctx, hmac );
 }
 
 /**
@@ -2263,7 +2256,7 @@ static void tls_hmac ( struct tls_cipherspec *cipherspec,
 		       uint64_t seq, struct tls_header *tlshdr,
 		       const void *data, size_t len, void *hmac ) {
 	struct digest_algorithm *digest = cipherspec->suite->digest;
-	uint8_t ctx[digest->ctxsize];
+	uint8_t ctx[ hmac_ctxsize ( digest ) ];
 
 	tls_hmac_init ( cipherspec, ctx, seq, tlshdr );
 	tls_hmac_update ( cipherspec, ctx, data, len );
@@ -2545,7 +2538,7 @@ static int tls_new_ciphertext ( struct tls_connection *tls,
 	struct tls_cipherspec *cipherspec = &tls->rx_cipherspec;
 	struct cipher_algorithm *cipher = cipherspec->suite->cipher;
 	struct digest_algorithm *digest = cipherspec->suite->digest;
-	uint8_t ctx[digest->ctxsize];
+	uint8_t ctx[ hmac_ctxsize ( digest ) ];
 	uint8_t verify_mac[digest->digestsize];
 	struct io_buffer *iobuf;
 	void *mac;
diff --git a/src/tests/hmac_test.c b/src/tests/hmac_test.c
index 871926f..5267999 100644
--- a/src/tests/hmac_test.c
+++ b/src/tests/hmac_test.c
@@ -100,26 +100,22 @@ struct hmac_test {
 static void hmac_okx ( struct hmac_test *test, const char *file,
 		       unsigned int line ) {
 	struct digest_algorithm *digest = test->digest;
-	uint8_t ctx[digest->ctxsize];
+	uint8_t ctx[ hmac_ctxsize ( digest ) ];
 	uint8_t hmac[digest->digestsize];
-	uint8_t key[test->key_len];
-	size_t key_len;
 
 	/* Sanity checks */
+	okx ( sizeof ( ctx ) == ( digest->ctxsize + digest->blocksize ),
+	      file, line );
 	okx ( test->expected_len == digest->digestsize, file, line );
 
-	/* Create modifiable copy of key */
-	memcpy ( key, test->key, test->key_len );
-	key_len = test->key_len;
-
 	/* Calculate HMAC */
 	DBGC ( test, "HMAC-%s key:\n", digest->name );
 	DBGC_HDA ( test, 0, test->key, test->key_len );
 	DBGC ( test, "HMAC-%s data:\n", digest->name );
 	DBGC_HDA ( test, 0, test->data, test->data_len );
-	hmac_init ( digest, ctx, key, &key_len );
+	hmac_init ( digest, ctx, test->key, test->key_len );
 	hmac_update ( digest, ctx, test->data, test->data_len );
-	hmac_final ( digest, ctx, key, &key_len, hmac );
+	hmac_final ( digest, ctx, hmac );
 	DBGC ( test, "HMAC-%s result:\n", digest->name );
 	DBGC_HDA ( test, 0, hmac, sizeof ( hmac ) );
 
diff --git a/src/tests/pccrc_test.c b/src/tests/pccrc_test.c
index f4ab573..e694932 100644
--- a/src/tests/pccrc_test.c
+++ b/src/tests/pccrc_test.c
@@ -467,11 +467,10 @@ peerdist_info_passphrase_okx ( struct peerdist_info_segment_test *test,
 			       uint8_t *pass, size_t pass_len,
 			       const char *file, unsigned int line ) {
 	struct digest_algorithm *digest = info->digest;
-	uint8_t ctx[digest->ctxsize];
+	uint8_t ctx[ hmac_ctxsize ( digest ) ];
 	uint8_t secret[digest->digestsize];
 	uint8_t expected[digest->digestsize];
 	size_t digestsize = info->digestsize;
-	size_t secretsize = digestsize;
 
 	/* Calculate server secret */
 	digest_init ( digest, ctx );
@@ -479,11 +478,9 @@ peerdist_info_passphrase_okx ( struct peerdist_info_segment_test *test,
 	digest_final ( digest, ctx, secret );
 
 	/* Calculate expected segment secret */
-	hmac_init ( digest, ctx, secret, &secretsize );
-	assert ( secretsize == digestsize );
+	hmac_init ( digest, ctx, secret, digestsize );
 	hmac_update ( digest, ctx, test->expected_hash, digestsize );
-	hmac_final ( digest, ctx, secret, &secretsize, expected );
-	assert ( secretsize == digestsize );
+	hmac_final ( digest, ctx, expected );
 
 	/* Verify segment secret */
 	okx ( memcmp ( test->expected_secret, expected, digestsize ) == 0,