Possible WebCrypto parameter choices

## RECOMMENDED values for parameters:
Based in part on RFC 6379 and SP800-57, aiming for 128-bit security level
<http://tools.ietf.org/html/rfc6379#section-3.2>
<http://csrc.nist.gov/publications/nistpubs/800-57/sp800-57_part1_rev3_general.pdf>

### Meta-values

[empty]     The empty string or octet string, or absent for optional parameters
[random]    A psuedo-random value
[p_3072]    Generator for 3072-bit MODP group [RFC3526]


### Key Generation

RsaKeyGenParams         modulusLength       3072
RsaKeyGenParams         publicExponent      0x010001
EcKeyGenParams          namedCurve          P-256
AesKeyGenParams         length              128
DhKeyGenParams          prime               [p_3072]
DhKeyGenParams          generator           2


### Signature / MAC

RsaSsaParams            hash                "SHA-256"
RsaPssParams            hash                "SHA-256"
RsaPssParams            saltLength          256
EcdsaParams             hash                "SHA-256"
HmacParams              hash                "SHA-256"


### Encryption

RsaOaepParams           hash                "SHA-256"
RsaOaepParams           label               [empty]
AesCtrParams            counter             [random]
AesCtrParams            length              32
AesCbcParams            iv                  [random]    <--- Req: Unpredictable
AesCfbParams            iv                  [random]    <--- Req: Unpredictable
AesGcmParams            iv                  [random]    <--- Req: Unique to each execution
AesGcmParams            additionalData      [empty]
AesGcmParams            tagLength           128


### Key Agreement

EcdhKeyDeriveParams     public              *****       <--- Impossible to auto-fill
DhKeyDeriveParams       public              *****       <--- Impossible to auto-fill


### Key Derivation

ConcatParams            hash                "SHA-256"
ConcatParams            algorithmId         [alg_name]  <--- From deriveKey invocation
ConcatParams            partyUInfo          [random]    <--- Not SP800-56A compliant, but as close as possible without ID_U
ConcatParams            partyVInfo          [random]    <--- Not SP800-56A compliant, but as close as possible without ID_V
ConcatParams            publicInfo          [empty]
ConcatParams            privateInfo         [empty]
HkdfCtrParams           hashAlg             "SHA-256"
HkdfCtrParams           label               [empty]
HkdfCtrParams           context             [empty]
Pbkdf2Params            salt                [urandom]
Pbkdf2Params            iterations          4096
Pbkdf2Params            prf                 "SHA-256"
Pbkdf2Params            password            *****       <--- Impossible to auto-fill