Prevent empty algorithm panic in CGO bindings and simplify SecretBox memory handling

[atulpatildbz]

Background

In google/go-tpm-tools#990, two related issues were identified and addressed in the Key Custody Core / keymanager components:

1. Slice index out of bounds in CGO bindings
   In both key_protection_service/key_custody_core/kps_key_custody_core_cgo.go (GenerateKEMKeypair) and workload_service/key_custody_core/ws_key_custody_core_cgo.go (GenerateBindingKeypair), proto.Marshal(algo) can produce an empty byte slice if an empty HpkeAlgorithm struct is passed.
   Attempting to pass (*C.uint8_t)(unsafe.Pointer(&algoBytes[0])) panics with a runtime slice bounds out of range error when len(algoBytes) == 0.

   Proposed Fix: Check for empty slice prior to the CGO call:

   if len(algoBytes) == 0 {
       return uuid.Nil, nil, fmt.Errorf("no algorithm provided")
   }

2. Simplify SecretBox::new in km_common/src/crypto/secret_box.rs
   Currently, SecretBox::new branches on data.capacity() > data.len():

   pub fn new(mut data: Vec<u8>) -> Self {
       let boxed: Box<[u8]> = if data.capacity() > data.len() {
           let b = Box::from(data.as_slice());
           data.zeroize();
           b
       } else {
           data.into_boxed_slice()
       };
       Self(secrecy::SecretBox::new(boxed))
   }

   As noted during code review on PR #990, branching on capacity() > len() introduces unnecessary branch duplication. Unconditional copy-then-zeroize is correct and consistent for both cases, avoiding any edge cases with reallocation or freed memory:

   pub fn new(mut data: Vec<u8>) -> Self {
       let boxed: Box<[u8]> = data.as_slice().into();
       data.zeroize();
       Self(secrecy::SecretBox::new(boxed))
   }

Affected Files

• key_protection_service/key_custody_core/kps_key_custody_core_cgo.go
• workload_service/key_custody_core/ws_key_custody_core_cgo.go
• km_common/src/crypto/secret_box.rs