Convert.FromBase64XYZ decoder should reject input when unused bits are not set to 0

[Copilot]

Summary

Implements RFC 4648 Section 3.5 compliance by rejecting Base64 input where unused bits are not set to zero. This ensures that decoding is deterministic—only one valid encoding exists for each byte sequence.

Fixes #105262

Problem

When the input is not a multiple of 3 bytes, some bits of the encoded values are not used. The encoder sets these bits to 0, but the decoder was not validating this constraint, allowing multiple different encoded strings to decode to the same value.

For example, when encoding a single byte [65] (ASCII 'A'), the encoder produces "QQ==". However, the decoder previously accepted 16 different variations ("QQ==", "QR==", "QS==", ..., "Qf=="), all decoding to the same value because the last 4 bits of the second character were not validated.

// Before this change:
Convert.FromBase64String("QQ==")  // Returns [65] ✓
Convert.FromBase64String("QR==")  // Returns [65] ✗ (should throw)
Convert.FromBase64String("Qf==")  // Returns [65] ✗ (should throw)

// After this change:
Convert.FromBase64String("QQ==")  // Returns [65] ✓
Convert.FromBase64String("QR==")  // Throws FormatException ✓
Convert.FromBase64String("Qf==")  // Throws FormatException ✓

Changes

Updated Convert.Base64.cs to validate unused bits in the TryDecodeFromUtf16 method:

• When 1 padding character (=): validates last 2 bits of the 3rd character are 0
• When 2 padding characters (==): validates last 4 bits of the 2nd character are 0

This brings Convert.FromBase64XYZ methods in line with the existing validation in Base64DecoderHelper.cs (used by System.Buffers.Text.Base64 and System.Buffers.Text.Base64Url).

RFC 4648 Section 3.5

"In some circumstances, the use of padding ("=") in base-encoded data is not required or used... implementations MUST reject the encoded data if it contains characters outside the base alphabet when interpreting base-encoded data."

The specification states that unused bits MUST be set to zero by conforming encoders, and decoders MAY reject input if these bits are not zero. This change enforces that rejection.

Testing

• Added 3 new tests to validate the unused bits rejection behavior
• Updated 2 existing tests that relied on accepting invalid encodings
• Fixed 1 test data value with invalid encoding
• All 60,417+ tests passing across System.Runtime.Extensions and System.Memory

Breaking Change

This is a breaking change, but justified because:

1. Encoders already comply: Convert.ToBase64XYZ methods always set unused bits to 0
2. RFC compliance: This implements the RFC 4648 specification correctly
3. Deterministic decoding: Ensures one canonical encoding per byte sequence
4. Limited impact: Only affects invalid/corrupted Base64 data or tests that randomly generate Base64 without proper validation
5. Consistency: Matches behavior of System.Buffers.Text.Base64 and Base64Url decoders

Original prompt

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This section details on the original issue you should resolve

<issue_title>Convert.FromBase64XYZ(...) decoder should reject input when unused bits are not set to 0 </issue_title>
<issue_description>When the input is not multiple of 3 some bits of the encoded values are not used, Convert.ToBase64XYZ(...) encoder overloads sets those bits to 0, but Convert.FromBase64XYZ(...) decoder overloads currently doesn't check those bits and allows any combination of values. Therefore multiple input could decoded to same value, for example when the input is 1 byte character 'A', encoder encodes it to 2 base64 characters and 2 padding "QQ==" , the last 4 bits of the 2nd Q is not used and set to 0s, but decoder doesn't validate that and allows 2^4 = 16 values decoded into same value, for example: "QQ==", "QR==", "QS==", "QT==", "QV==", "QU==", "QW==", "QX==", "QY==", "QZ==", "Qa==", "Qb==", "Qc==", "Qd==", "Qe==", "Qf" will be decoded to a same value, 65, ascii of 'A'.

The spec mentions that unused bits MUST be set to zero by conforming encoders. It also mentions that decoders may reject an input if pad bits have not been set to zero. Don't see any reason to keep allowing non-zero value for those other combinations that produce same result when encoders are expected to produce only one value.

This doesn't seem to be a breaking change, when encoders expected to set unused bits to 0. Though it could break tests that randomly generates Base64 encoded text for decoding.

Related to #105262</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #105264

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[Copilot]

Pull Request Overview

This PR implements RFC 4648 Section 3.5 compliance by making Convert.FromBase64XYZ methods reject Base64 input where unused bits are not set to zero, ensuring deterministic decoding where only one valid encoding exists for each byte sequence.

Key Changes:

• Modified validation logic in Convert.Base64.cs to check unused bits in Base64 padding scenarios
• Updated test cases to use valid encodings and added comprehensive tests for unused bit validation
• Fixed one test data value that contained an invalid encoding

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/System.Private.CoreLib/src/System/Convert.Base64.cs	Added bitwise checks to validate that unused bits are zero when padding is present
src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/Convert.FromBase64.cs	Replaced tests that relied on invalid encodings with valid ones, and added new tests to verify unused bit rejection behavior
src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/Convert.cs	Corrected test data to use a valid Base64 encoding with zero unused bits

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