Khaos Wallet Core is an open source, cross-platform, mobile-focused library implementing low-level cryptographic wallet functionality for a high number of blockchains. It is a core part of the popular (https://khaoswallet.com), and some other projects. Most of the code is C++ with a set of strict C interfaces, and idiomatic interfaces for supported languages: Swift for iOS and Java for Android.
Khaos Wallet Core is available under the MIT license. See the LICENSE file for more info.