fix(crypto): Use full 12-byte GCM nonce with random prefix

[justin-layerv]

Summary

• Fix GCM nonce to use all 12 bytes instead of only 8
• Add random 4-byte prefix to increase nonce entropy
• Maintain backward compatibility with existing implementations

Problem

The GCM nonce only used 8 of 12 bytes. Bytes 0-4 were always zero:

// Before (only 8 bytes used):
func (h *HeaderCurve) NonceBytes() []byte {
    var nonce [GCMNonceSize]byte  // 12 bytes, initialized to zero
    copy(nonce[4:12], h.HeaderCommon[16:24])  // Only 8 bytes!
    return nonce[:]  // nonce[0:4] always zero
}

This reduced effective nonce entropy and made patterns more predictable.

Solution

Use HeaderCommon bytes 12-16 (previously unused) as a random nonce prefix:

// After (all 12 bytes used):
func (h *HeaderCurve) NonceBytes() []byte {
    var nonce [GCMNonceSize]byte
    copy(nonce[0:4], h.HeaderCommon[12:16])  // Nonce prefix (4 bytes)
    copy(nonce[4:12], h.HeaderCommon[16:24]) // Counter (8 bytes)
    return nonce[:]
}

HeaderCommon Layout (24 bytes)

Bytes	Field	Usage
0-4	Preamble	Random
4-8	Type/Size	XOR'd with preamble
8-10	Version	Major/Minor
10-12	Flags	Protocol flags
12-16	Nonce Prefix	NEW: Random prefix
16-24	Counter	Transaction ID

Files Modified

File	Change
nhp/core/scheme/curve/header.go	Add SetNoncePrefix(), fix NonceBytes()
nhp/core/scheme/gmsm/header.go	Add SetNoncePrefix(), fix NonceBytes()
nhp/core/packet.go	Add SetNoncePrefix() to Header interface
nhp/core/initiator.go	Call SetNoncePrefix() with random value
nhp/core/scheme/curve/header_test.go	Add nonce verification tests

Backward Compatibility

• Old senders: bytes 12-15 are zero → still works, just less entropy
• New senders: bytes 12-15 are random → full entropy
• No protocol version bump needed - responders just read bytes as-is

Test Plan

• TestNonceBytesLength - Verify 12 bytes returned
• TestNonceBytesContent - Verify prefix and counter placement
• TestNonceNoZeroGap - Verify no zero gaps when values set
• TestNonceUniqueness - Verify different inputs produce unique nonces

[craftleon]

Header bytes [12:16] are reserved in purpose. For opennhp project, prefer not use them as nonce prefix.

[craftleon]

Change declined