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Fix spec compliant record size verification #5

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Fix spec compliant record size verification #5

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fbfc75d
Fix spec compliant record size verification
p1gp1g Feb 4, 2025
a424e1d
Fix decryption fail with non-constant recordSize
p1gp1g Feb 4, 2025
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7 changes: 7 additions & 0 deletions webpush/src/main/java/com/google/crypto/tink/apps/webpush/WebPushConstants.java
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Original file line number Diff line number Diff line change
Expand Up @@ -63,6 +63,13 @@ final class WebPushConstants {
static final int CIPHERTEXT_OVERHEAD =
CONTENT_CODING_HEADER_SIZE + PADDING_DELIMETER_SIZE + DEFAULT_PADDING_SIZE + TAG_SIZE;

// https://www.rfc-editor.org/rfc/rfc8188#section-2.1
// > rs: Values smaller than 18 are invalid.
//
// * min data length: 1
// * padding delimeter: 1
// * AES-GCM tag size: 16
static final int MIN_RS = 18;
static final int MAX_CIPHERTEXT_SIZE = 4096;

static final String HMAC_SHA256 = "HMACSHA256";
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12 changes: 8 additions & 4 deletions webpush/src/main/java/com/google/crypto/tink/apps/webpush/WebPushHybridDecrypt.java
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Expand Up @@ -111,7 +111,10 @@ private WebPushHybridDecrypt(Builder builder) throws GeneralSecurityException {
}
this.authSecret = builder.authSecret;

if (builder.recordSize < WebPushConstants.CIPHERTEXT_OVERHEAD
// Nothing prevent a max bound for rs. But we know that push messages
// are at most MAX_CIPHERTEXT_SIZE (0x1000), and it is usually set to
// this value for convenience.
if (builder.recordSize < WebPushConstants.MIN_RS
|| builder.recordSize > WebPushConstants.MAX_CIPHERTEXT_SIZE) {
throw new IllegalArgumentException(
String.format(
Expand Down Expand Up @@ -228,9 +231,10 @@ public byte[] decrypt(final byte[] ciphertext, final byte[] contextInfo /* unuse
record.get(salt);

int recordSize = record.getInt();
if (recordSize != this.recordSize
|| recordSize < ciphertext.length
|| recordSize > WebPushConstants.MAX_CIPHERTEXT_SIZE) {
// Fail if the recordSize is too low for the ciphertext,
// or higher than the recordSize we defined
if (recordSize > this.recordSize
|| recordSize < (ciphertext.length - WebPushConstants.CONTENT_CODING_HEADER_SIZE)) {
throw new GeneralSecurityException("invalid record size: " + recordSize);
}

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5 changes: 3 additions & 2 deletions webpush/src/main/java/com/google/crypto/tink/apps/webpush/WebPushHybridEncrypt.java
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Original file line number Diff line number Diff line change
Expand Up @@ -134,7 +134,7 @@ public Builder() {}
*/
@CanIgnoreReturnValue
public Builder withRecordSize(int val) {
if (val < WebPushConstants.CIPHERTEXT_OVERHEAD
if (val < WebPushConstants.MIN_RS
|| val > WebPushConstants.MAX_CIPHERTEXT_SIZE) {
throw new IllegalArgumentException(
String.format(
Expand Down Expand Up @@ -216,7 +216,8 @@ public byte[] encrypt(final byte[] plaintext, final byte[] contextInfo /* unused
throw new GeneralSecurityException("contextInfo must be null because it is unused");
}

if (plaintext.length > recordSize - paddingSize - WebPushConstants.CIPHERTEXT_OVERHEAD) {
// rs >= len(plaintext + AES-GCM tag + padding delimiter + padding)
if (plaintext.length > recordSize - paddingSize - WebPushConstants.TAG_SIZE - 1) {
throw new GeneralSecurityException(
String.format(
"plaintext too long; with record size = %d and padding size = %d, plaintext cannot"
Expand Down