[crypto] Generalise rsa_parse_integer() to asn1_enter_unsigned()

ECDSA signature values and private keys are fixed-length unsigned
integers modulo N (the group order of the elliptic curve) and are
therefore most naturally represented in ASN.1 using ASN1_OCTET_STRING.

Private key representations do use ASN1_OCTET_STRING, but signature
values tend to use ASN1_INTEGER, which adds no value but does ensure
that the encoding becomes variable-length and requires handling a
pointless extra zero byte if the MSB of the unsigned value happens to
be set.

RSA also makes use of ASN1_INTEGER for modulus and exponent values.
Generalise the existing rsa_parse_integer() to asn1_enter_unsigned()
to allow this code to be reused for ECDSA.

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

2 files changed, 27 insertions, 30 deletions

diff --git a/src/crypto/asn1.c b/src/crypto/asn1.c
index 4ee95415f..819a8aadb 100644
--- a/src/crypto/asn1.c
+++ b/src/crypto/asn1.c
@@ -373,6 +373,29 @@ int asn1_enter_bits ( struct asn1_cursor *cursor, unsigned int *unused ) {
 }
 
 /**
+ * Enter ASN.1 unsigned integer
+ *
+ * @v cursor		ASN.1 object cursor
+ * @ret rc		Return status code
+ */
+int asn1_enter_unsigned ( struct asn1_cursor *cursor ) {
+	int rc;
+
+	/* Enter integer */
+	if ( ( rc = asn1_enter ( cursor, ASN1_INTEGER ) ) != 0 )
+		return rc;
+
+	/* Skip initial positive sign byte if applicable */
+	if ( ( cursor->len > 1 ) &&
+	     ( *( ( uint8_t * ) cursor->data ) == 0x00 ) ) {
+		cursor->data++;
+		cursor->len--;
+	}
+
+	return 0;
+}
+
+/**
  * Parse value of ASN.1 boolean
  *
  * @v cursor		ASN.1 object cursor
diff --git a/src/crypto/rsa.c b/src/crypto/rsa.c
index 14456f755..9c0982cf6 100644
--- a/src/crypto/rsa.c
+++ b/src/crypto/rsa.c
@@ -138,34 +138,6 @@ static int rsa_alloc ( struct rsa_context *context, size_t modulus_len,
 }
 
 /**
- * Parse RSA integer
- *
- * @v integer		Integer to fill in
- * @v raw		ASN.1 cursor
- * @ret rc		Return status code
- */
-static int rsa_parse_integer ( struct asn1_cursor *integer,
-			       const struct asn1_cursor *raw ) {
-
-	/* Enter integer */
-	memcpy ( integer, raw, sizeof ( *integer ) );
-	asn1_enter ( integer, ASN1_INTEGER );
-
-	/* Skip initial sign byte if applicable */
-	if ( ( integer->len > 1 ) &&
-	     ( *( ( uint8_t * ) integer->data ) == 0x00 ) ) {
-		integer->data++;
-		integer->len--;
-	}
-
-	/* Fail if cursor or integer are invalid */
-	if ( ! integer->len )
-		return -EINVAL;
-
-	return 0;
-}
-
-/**
  * Parse RSA modulus and exponent
  *
  * @v modulus		Modulus to fill in
@@ -226,7 +198,8 @@ static int rsa_parse_mod_exp ( struct asn1_cursor *modulus,
 	}
 
 	/* Extract modulus */
-	if ( ( rc = rsa_parse_integer ( modulus, &cursor ) ) != 0 )
+	memcpy ( modulus, &cursor, sizeof ( *modulus ) );
+	if ( ( rc = asn1_enter_unsigned ( modulus ) ) != 0 )
 		return rc;
 	asn1_skip_any ( &cursor );
 
@@ -235,7 +208,8 @@ static int rsa_parse_mod_exp ( struct asn1_cursor *modulus,
 		asn1_skip ( &cursor, ASN1_INTEGER );
 
 	/* Extract publicExponent/privateExponent */
-	if ( ( rc = rsa_parse_integer ( exponent, &cursor ) ) != 0 )
+	memcpy ( exponent, &cursor, sizeof ( *exponent ) );
+	if ( ( rc = asn1_enter_unsigned ( exponent ) ) != 0 )
 		return rc;
 
 	return 0;