Accept zero-length ciphertext in TSIP AES-GCM decrypt

[miyazakh]

Description

wc_tsip_AesGcmDecrypt() rejected sz == 0, so AAD-only (GMAC) decrypt failed.
It now accepts a zero-length ciphertext and skips the zero-size buffer
allocation and copies, matching wc_tsip_AesGcmEncrypt().

Testing

Added zero-length decrypt cases to tsip_aesgcm_zerolen_test():

• empty payload with AAD, round trip
• corrupted tag must fail
• empty payload and empty AAD, round trip

Verified on RX72N EnvisionKit (TSIP); all TSIP unit tests pass.

Checklist

• added tests
• updated/added doxygen
• updated appropriate READMEs
• Updated manual and documentation

[Copilot]

Copilot review overview

🟡 Changes recommended

The new behavior is tested only with AES-128 despite using distinct TSIP AES-256 entry points.

Review effort: Balanced
Findings: 1

Open (1)

• Add AES-256 zero-length coverage for TSIP GCM · New

What changed in this PR

Enables TSIP AES-GCM decryption for zero-length ciphertexts, supporting GMAC/AAD-only operations.

Changes:

• Allows null input/output when ciphertext length is zero.
• Avoids zero-size allocations and copies.
• Adds round-trip and corrupted-tag tests.

File	Description
wolfcrypt/​src/​port/​Renesas/​renesas_tsip_aes.c	Handles zero-length TSIP AES-GCM decrypt input.
IDE/​Renesas/​e2studio/​RX72N/​EnvisionKit/​wolfssl_demo/​wolfssl_tsip_unit_test.c	Adds zero-length AES-GCM regression cases.

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[github-actions]

MemBrowse Memory Report

gcc-arm-cortex-m4-openssl-compat

• FLASH: .text +128 B (+0.0%, 792,412 B / 1,048,576 B, total: 76% used)

gcc-arm-cortex-m4-pkcs7

• FLASH: .text +64 B (+0.0%, 221,790 B / 262,144 B, total: 85% used)

gcc-arm-cortex-m4-pq

• FLASH: .text +64 B (+0.0%, 308,656 B / 1,048,576 B, total: 29% used)

gcc-arm-cortex-m7

• FLASH: .text +64 B (+0.0%, 207,852 B / 262,144 B, total: 79% used)

linuxkm-standard

• Data: __patchable_function_entries +8 B (+0.0%, 51,600 B)
  No memory changes detected for:
• gcc-arm-cortex-m0plus
• gcc-arm-cortex-m3
• gcc-arm-cortex-m4
• gcc-arm-cortex-m4-baremetal
• gcc-arm-cortex-m4-crypto-only
• gcc-arm-cortex-m4-dtls13
• gcc-arm-cortex-m4-min-ecc
• gcc-arm-cortex-m4-rsa-only
• gcc-arm-cortex-m4-sp-math
• gcc-arm-cortex-m4-tls12
• gcc-arm-cortex-m4-tls13
• gcc-arm-cortex-m7-pq
• gcc-arm-cortex-m7-tls13
• linuxkm-pie
• stm32-sim-stm32h753