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Adjust minimum size of PKCS#1 RSA padding overhead.

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This raises the space overhead estimate by one byte, due to me having
originally forgotten that when signing using RSA PKCS#1 padding
scheme, the plaintext of the RSA operation needs a leading 0 byte.

Bug: 21955742
Change-Id: If78efcd375afd8902a14a2217ddedfea87d4d337
This commit is contained in:
Alex Klyubin
2015-06-22 13:21:59 -07:00
parent c54bcade1b
commit 6e90ade5dd
1 changed files with 8 additions and 7 deletions
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15
keystore/java/android/security/keystore/AndroidKeyStoreKeyPairGeneratorSpi.java
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@@ -624,9 +624,8 @@ public abstract class AndroidKeyStoreKeyPairGeneratorSpi extends KeyPairGenerato
// Constraints:
// 1. Key must be authorized for signing without user authentication.
// 2. Signature digest must be one of key's authorized digests.
// 3. For RSA keys, the digest output size must not exceed modulus size minus space needed
// for RSA PKCS#1 signature padding (about 29 bytes: minimum 10 bytes of padding + 15--19
// bytes overhead for encoding digest OID and digest value in DER).
// 3. For RSA keys, the digest output size must not exceed modulus size minus space overhead
// of RSA PKCS#1 signature padding scheme (about 30 bytes).
// 4. For EC keys, the there is no point in using a digest whose output size is longer than
// key/field size because the digest will be truncated to that size.
@@ -727,10 +726,12 @@ public abstract class AndroidKeyStoreKeyPairGeneratorSpi extends KeyPairGenerato
spec.getDigests(),
AndroidKeyStoreBCWorkaroundProvider.getSupportedEcdsaSignatureDigests());
// The amount of space available for the digest is less than modulus size because
// padding must be at least 10 bytes long, and then there's also the 15--19
// bytes overhead for encoding digest OID and digest value in DER.
int maxDigestOutputSizeBits = keySizeBits - 29 * 8;
// The amount of space available for the digest is less than modulus size by about
// 30 bytes because padding must be at least 11 bytes long (00 || 01 || PS || 00,
// where PS must be at least 8 bytes long), and then there's also the 15--19 bytes
// overhead (depending the on chosen digest) for encoding digest OID and digest
// value in DER.
int maxDigestOutputSizeBits = keySizeBits - 30 * 8;
int bestKeymasterDigest = -1;
int bestDigestOutputSizeBits = -1;
for (int keymasterDigest : availableKeymasterDigests) {