W3C WebCrypto for cloud
However, the library isn't confined to IE11: the company says it's been tested on all Internet Explorer versions from IE8 up, as well as the current Firefox, Chrome, Opera and Safari browsers.
Supported crypto functions in the library include RSA PKCS#1 v1.5, OAEP and PSS; its psuedo-random number generator (PRNG) follows the AES-CTR spec; and NIST's Concat KDF (key derivation function).
The big number integer arithmetic in the library, which includes “addition, subtraction, multiplication, division, reduction, inversion, GCD, extended Euclidean algorithm (EEA), Montgomery multiplication, and modular exponentiation”, and MS notes that this library will probably be replaced in future versions.
MS notes that most of the W3C WebCrypto API – here if you're interested – is supported, with the exception of “promises”.
Since it's still under development, the library should probably be used with the usual “at own risk” caveat for now. ®