simple_app protect_blob() fails w/--public_key_alg=rsa-3072 --auth_symmetric_key_alg=aes-256-gcm

[gapisback]

Testing with test case run_test-crypto_algorithms added by PR #232 , simple_app fails to succeed when run with this pair of crypto-algorithms: '--public_key_alg=rsa-3072 --auth_symmetric_key_alg=aes-256-gcm'

run_example.sh: Test case 2: simple_app with public_key algorithm='rsa-3072' and symmetric-key algorithm='aes-256-gcm'


**************************************************************************************************************
run_example.sh: simple_app: Running run_simple_app_as_server_talk_to_Cert_Service
+ set +x

**************************************************************************************************************
run_example.sh: simple_app: Running run_app_by_name_as_server_talk_to_Cert_Service
+ set +x
run_example.sh: Public-key algorithm: '--public_key_alg=rsa-3072', Authenticated Symmetric-key algorithm: '--auth_symmetric_key_alg=aes-256-gcm'

+ /home/agurajada/Code/certifier-framework-for-confidential-computing/sample_apps/simple_app/example_app.exe --data_dir=./app2_data/ --operation=cold-init --measurement_file=example_app.measurement --policy_store_file=policy_store --public_key_alg=rsa-3072 --auth_symmetric_key_alg=aes-256-gcm --print_all=true
measurement file='example_app.measurement', public_key_alg='rsa-3072', authenticated_symmetric_key_alg='aes-256-gcm
protect_blob() error, line 862, protect_blob: key too small
save_store() error, line 637, can't protect blob
cold_init() error, line 1355, Can't save store
main() error, line 432, cold-init failed

While developing the list of supported Authenticated symmetric-key algorithms, during code review of PR #233 , it was confirmed that this Enc_method_aes_256_gcm = "aes-256-gcm" should be a valid crypto-algorithm name.

But testing shows some other deficiencies. Either we have to fix the logic in generate_symmetric_key() or re-visit whether this "aes-256-gcm" is a valid Authenticated symmetric-key algorithm.