The GSM speech service is secure up to the point where speech enters the core network but over the core network it has no security. In order to have an end-to-end security, speech must be encrypted before it enters the GSM network. The reason for not using encryption at the input of the GSM handset is that the encrypted speech signal would be randomised and would not exhibit the speech like characteristics on which the GSM speech transcoding principles are based. Hence die encoded signal would be significantly distorted by the GSM transcoding process, making the resultant signal unsuitable for the decryption process. Here we present a method that allows encryption at the input to the GSM terminal by modulating the encrypted data onto speech-like waveforms, so that it goes through the GSM system with sufficient accuracy to be decoded and decrypted at the receiving end. Throughputs of 3 and 4 kb/s have been achieved with the modulation introducing 0.5% and 4% bit error rates (BER) respectively. With the addition of error correcting codes near zero BER has been achieved at 1.8 kb/s, allowing the robust transmission of the bitstream from a modem low-bit rate speech coder. (4 pages)