By train: If you are coming from Eurocrypt in Barcelona, taking a train is a sensible option (even though flying can be cheaper if you book well in advance!). There are several connections a day, you can check timetable and make your reservations at RENFE.
By Plane: You can get low cost flights Barcelona-Madrid from Spanish companies (Vueling, Air Madrid). Also Iberia and Spanair offer very fair rates if you book well enough in advance.
Barajas Airport is the only commercial airport in Madrid, and is located in the outskirts of the city. It i has four different terminals:
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The older terminals T1, T2 and T3, from which you may reach Madrid by either subway (Line 8) or taxi. Subway is strongly recommended: it is fast, cheap, easy to use and allows an easy access to every part of the city. If you prefer a taxi it will be a bit more expensive, but it is still an excellent option without any remarkable inconvenience. There is also a public bus that goes to the Colon Square in the center of Madrid, but depending on the location your hotel this may be not a flexible choice.
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Otherwise, if you arrive to the new terminal T4, before going to Madrid you must first go to the older terminals building and follow the recommendations in the previous item. The connection between T4 and the other terminals is done through a free shuttle train that has a high frequency (about 3 min.).
For further information (in either Spanish or English) visit AENA web page (select Madrid-Barajas airport).